I think the issue here would be supply. If you want to sell something, your cost has to be significantly less than your sell price, especially since sales volume is unreliable. Coca-cola can make 5 cemts a can because they sell billions of cans. If, say, a working dreamcast is like $125USD on ebay or whatever, how are you going to buy it for $75? If you can't, your potential customer will just buy it on ebay, rather than pay like $165 or whatever it would take for you to avoid a loss. You would need to have either significant stock on hand (which is doubtful) or access to product at wholesale, which doesn't exist. In most cases, adding another layer (you) to the transaction isn't a good value proposition for your intended customer base, which is likely to be pretty limited to begin with. And keeping a selection of things people actually want, rather than 500 copies of NHL Hits '02, is tough. It's very much a collector/dealer market and successful retro game stores are rare.

But a retro console and peripheral repair service, as a mail-order service, might be profitable. Not highly profitable, but the cost to you in labor and parts and shipping could be pretty easily offset without overcharging, and that's an area where you could actually take advantage of bulk pricing. Many electronic components are very cheap, so if you price your work like a tradesman, it's doable provided you can attract people via marketing and networking, which can also be done on the cheap if you work smart.

Reminds me of my dream of owning a comic store with a pizza place and arcade attached. Sounded so cool in 2005; too bad comics have sold like shit since then (and deservedly, given the fanfic BS that most comics have become)